1. Field of the Invention
The present invention relates to an image forming apparatus that forms a toner image on a sheet of paper, and more particularly, to a technique for collecting toner from a photoconductive surface of a photoconductor.
2. Description of the Related Art
While toner has been becoming smaller in size and more spherical in shape as the image quality of an image forming apparatus is upgraded, application of the traditional blade cleaning has been becoming difficult. Such being the case, so-called cleanerless process that does not use a blade has been attracting attention.
According to the cleanerless process in a color image forming apparatus of the so-called tandem system, when an image transferred onto a sheet of paper or an intermediate transfer body in an image forming station on the upstream side reaches the non-image portion of the photoconductor in an image forming station at the latter stage, an event that part of the image adheres onto the photoconductor, known as “inverse transfer” phenomenon, may possibly take place. Toner transferred onto a sheet of paper in the image forming station on the upstream side is inversely transferred onto the photoconductor at the transfer position in the image forming station at the latter stage, and collected in a developer in the latter stage without being subjected to blade cleaning. This gives rise to so-called color mixing phenomenon that the color tone of toner in the developer in the image forming station at the latter stage changes gradually. Although the degree of change varies with the kinds or patterns of an image to be printed, it is still a fundamental problem of the cleanerless process in the color image forming apparatus of the tandem system.
Generally, the polarity of transfer residual toner of the normal color remaining on the photoconductor, which has been developed but has not been transferred in the transfer process in one image forming station, is the normal polarity of the toner, whereas the mixed color toner remaining on the photoconductor after the transferring, which has been transferred onto a sheet of paper in the image forming station on the upstream side and inversely transferred onto the photoconductor at the transfer position in the above-specified image forming station, is often charged to a reversed polarity to the normal polarity.
Given these circumstances, a technique for solving the color mixing problem of toner as described above has been disclosed (see JP-A-2000-242152).
The related art described above provides “inversely transferred toner removing means” for selectively collecting inversely transferred toner migrating from the image forming station in the preceding stage, which is chiefly produced in the tandem system. According to this related art, a roller-shaped member (for example, a brush roller) used to remove inversely transferred toner is pressed against the photoconductor after the transferring and bias of the same polarity as the normal charged polarity of toner is applied, so that toner charged to the reversed polarity alone is selectively collected. In this instance, toner of the normal color is not collected, and instead it is collected in the developer by way of the charger and the exposure device after it is stirred to erase the memory pattern. The so-called cleanerless process is thus achieved.
The related art, however, has problems chiefly as follows.
That is, in the related art, it is possible to collect inversely transferred toner (that is, mixed color toner); however, transfer residual toner of the normal color is not collected, and instead it is collected in the developer after passing by the charger and the exposure device. Because it is the toner of the normal color, no color mixing occurs. Nevertheless, when a quantity of the transfer residual toner is large, the pattern is not erased in a satisfactory manner by stirring the transfer residual toner in the inverse transfer cleaner portion. This gives rise to irregular charging or shielding of light during exposure, and image formation in the following process is adversely affected.
In addition, in the related art described above, although being the inversely transferred toner, waste toner is constantly generated from it, and there is a need for a place to store collected waste toner. Providing a waste toner box separately from the cleaner portion raises the need for carrying means for carrying waste toner from the cleaner portion to the waste toner box, which complicates the apparatus configuration. Because the inversely transferred toner is generally far less than the transfer residual toner in quantity, the inverse transfer cleaner portion and the waste toner storing portion may be formed as one body and provided in the form of a cartridge. The waste toner storing portion, however, becomes larger in size when the life of the inverse transfer cleaner is long, which may possibly pose an obstacle to achieve a size reduction of the apparatus.